A planar lighting apparatus is used as backlight illumination in a liquid crystal display device used in a display panel or the like. In the above planar lighting apparatus, a discharge tube, light-emitting diodes (LEDs) and the like are generally used as a light source. Upon using the above planar lighting apparatus in a large-size display or the like, it is required to output light with high luminance and strong chromaticity. There is also required elaboration to make luminance uniform by uniformly irradiating light from the light source to a display panel surface to eliminate luminance nonuniformity on the entire display panel surface.
In recent years, study on the utilization of three laser light sources of red light (R light), green light (G light) and blue light (B light) has been started to obtain light with high luminance and strong chromaticity as a light source of a display device. As an exemplary construction for planarly emitting light from these laser light sources, the R, G and B light are combined into one laser light, which is then scanned by a polygon minor and reflected using a flat reflecting mirror (see, for example, patent literature 1). If the polygon mirror is used, scanning speed differs between end parts and a central part of a scanning range and this scanning speed difference increases as a scanning angle increases. The above scanning speed difference causes luminance nonuniformity, but the scanning speed is improved to be constant using an fθ lens in patent literature 1.
Another known planar lighting apparatus is such that light emitting elements such as LEDs are arranged on a side surface of a light guide plate and luminous fluxes from these LEDs are incident on the side surface of the light guide plate and reflected upward of the light guide plate (see, for example, patent literature 2). In patent literature 2, luminance nonuniformity is reduced by elaborating the shapes of a multitude of prisms provided on the lower surface of the light guide plate. Specifically, the directions or angles of inclination of slants of the prisms formed on the light guide plate at the incident side of the luminous fluxes change depending on the positions of the luminous fluxes on the incident surface. Alternatively, the prisms are formed such that the slants, valley lines or ridge lines thereof are concave with respect to the incident surface of the luminance fluxes.
Speed variation caused by the scanning of the polygon mirror is eliminated using the fθ lens in patent literature 1. If the fθ lens is used, the scanning angle of the polygon mirror cannot be increased, wherefore a distance between the polygon mirror and the fθ lens needs to be largely increased to cause a problem of enlarging the display device. Since an fθ lens needs to be accordingly enlarged in a large-size display, there is a problem of being extremely expensive.
In patent literature 2, a construction for improving the luminance nonuniformity to a sufficient level for the use as an illumination light source of a display device is not elucidated although it is indicated that the luminance nonuniformity of the planar lighting apparatus is improved. Accordingly, there is a problem of not elucidating a construction for obtaining sufficient luminance in a large-size display by a light guide plate method using LEDs as light sources.
[Patent Literature 1]
Japanese Unexamined Patent Publication No. H06-148635
[Patent Literature 2]
Japanese Unexamined Patent Publication No. 2006-155964